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Water catchments affected by Australia's most deadly bushfire disaster could be at risk from pollution for several years, say experts.

Forest hydrologist, Dr Patrick Lane of the University of Melbourne says the evidence comes from recent studies into the impact of earlier fires on catchments.

Forested areas usually have very high water quality because vegetation intercepts rain and reduces run-off and soil erosion, says Lane, who is also an author on a new Land & Water Australia report into the impacts of fire on water catchments.

He says the water that emerges from such areas normally percolates through the soil and is slowly released into water storage areas.

But, after a fire, the lack of vegetation to protect the soil can lead to massive erosion, and even "debris flows" that form "instant gullies".

"You get this blast of high intensity rainfall that gouges out large gullies," says Lane. "They'll dump a lot of sediment and nutrients and rocks and all sorts of stuff into streams."

He says a debris flow was responsible for the only death - of a fire-fighter - in the 2003 Victorian fires.

Sediment and algal blooms

Lane says sediment in run-off causes turbidity in water supplies and nutrients in the sediment later lead to algal blooms in lakes and dams.

"Areas are the most vulnerable in the first few months after a fire because there's very little regrowth," says Lane.

"One, two, three, four years is the danger period," he adds. "Normally you would expect the first year to be the worse but we have had some instances where a year after the fire, with quite good vegetation recovery, we had the worst erosion events."

Immediately after the Black Saturday fires earlier this year Melbourne water authorities transferred water from burnt catchments into unaffected reservoirs before rain fell, says Lane.

"In this case Melbourne has probably got off fairly lightly, although there is still some potential," he says.

But, Lane says, some of the regional areas badly affected by the fire, including Kilmore and Marysville, remain at the mercy of rain that may trigger pollution of water supplies.

"Their water supply would be in some jeopardy," says Lane.

He says it's hard to predict the exact impact because it will depend on the site and the amount and intensity of the rainfall.

Lane and other experts are advising water authorities on how best to protect catchments in the future, by considering storage dams to catch sediment, filtration plants, or alternative water sources.

Water yield affected

Lane says a further problem is the impact of bushfires on catchment yield, especially in Mountain Ash forests.

He says research shows water supplies can increase in the first five years after a fire in such catchments - because there is less water being pumped into the atmosphere by trees.

But this extra water will be carrying more sediment and can even lead to floods if rainfall is too intense, says Lane.

And in the long term, yield can fall as much as 25% as a growing forest soaks up more water than a mature forest.

Lane says one option to reduce the impact on yield could be to thin vegetation as it's growing, but there is a lack of evidence on what the long-term effects of this might be.

Some concern has been expressed about the impact of fire retardants on water quality but Lane says the studies he has been involved in did not study these.